1. Field of the Invention
This invention pertains to a flywheel assembly particularly adapted for energy storage applications, and more particularly pertains to construction of a three-dimensional, reinforced, composite, material, flywheel, outer portion and a hub with an interface for mechanically locking the hub and outer portion to one another.
2. Description of the Prior Art
Recently, interest in flywheels has been revived because flywheels can help solve contemporary problems of crucial importance. For example, three special areas where flywheels can help concern: (a) problems relating to an expanding use of energy; (b) problems relating to an impact of that use on the environment; and (c) problems relating to more efficient use of energy.
With respect to the solution of the above problems, flywheels offer the prospects of providing energy storage systems for use with: solar energy systems, mechanical power systems, and electrical power systems. Flywheels, for instance, may provide an efficient means of storing energy on a large scale to help electric utilities handle peak loads. Additionally, flywheels may store energy for both propulsion and auxiliary power of: air, land, sea and space vehicles (such as: trains, automobiles, trucks, buses, submarines, airplanes and space vehicles). Moreover, flywheels may provide compact units to power the above vehicles.
However, until recently, the use of prior art flywheels has been limited to a very few applications because of the following principal disadvantages. For example, prior art flywheels cannot store sufficient amounts of energy within practical weight and volume constraints. Further, high speed composite wheels are subject to delamination, and have inefficient geometric configurations and certain structural dynamic limitations.
The extent or seriousness of these disadvantages are essentially determined by three important characteristics of flywheels. Namely, the materials comprising the flywheel, the geometric configuration of the flywheel, and in the case of filament wound composites, the method of fabricating the flywheel.
This is so because flywheels operate in accordance with the following principles: First, the maximum amount of energy stored in a flywheel depends on: the mass of the flywheel, the distribution of the mass, and the maximum allowable rotational speed of the flywheel. Second, stored energy varies as the square of the rotational speed of the flywheel. And third, the maximum rotational speed is limited by the strength of the material of the flywheel. Hence, it follows that high strength to weight ratio is a key criteria for selecting material to fabricate high performance energy storing flywheels.
However, prior art flywheels have been traditionally made from materials which do not satisfactorily meet the above criteria, therefore they suffer the above named disadvantages. For example, high performance metal flywheels (even those made of the strongest alloyed steel) are generally unsatisfactory, notwithstanding their great strength, because most metal flywheels possess large densities of around 8 grams per cubic centimeters. These densities cause the metal flywheels to have a very large weight and volume in order to store sufficient energy. Additionally, utilization of metal flywheels at high stress levels increases the hazards of catastrophic failure because of a great increase of energy of failed pieces. Thus, as a consequence of the foregoing, metal flywheels will usually be limited to applications where either adequate failure protection can be provided, or where performance can be derated sufficiently to provide an adequate margin of safety.
Similarly, state of the art flywheels composed of typical reinforced composite materials or two-dimensional reinforced plastic materials, such as filament wound composite assemblies, are inadequate due to the low strength characteristics of the material in directions perpendicular to unidirectional fibers within the composite. Radial loads occur in directions perpendicular to the unidirectional fibers impose tensile loads on the resin matrix material. The resin, which is usually an epoxy resin, is very poor in withstanding these tensile loads. Thus, rotational loads cause delaminations at speeds below the limits set by the high tensile strength to weight ratio of the filament wound composite materials. Consequently, filament wound composite flywheel assemblies have yet to demonstrate the potential illustrated by the high strength to weight ratio of the composite material.
In an attempt to alleviate the problem of delamination, several approaches have been ventured, such as the utilization of uniform rods and thin hoops in which it is attempted to achieve a one dimensional stress field. However, these approaches have tended to avoid delamination problems at a severe penalty with respect to the energy stored per pound of material, or energy stored per unit volume.
In addition to the above, radial, axial, and shear stresses often cause the rim wheel, or outer portion of prior art composite flywheels to separate from the hub. This occurs because the bonded interface between the outer portion and the hub is not strong enough to accommodate radial, axial and shear stresses experienced in operational conditions without failure.
Lastly, designers are severely limited in their ability to alter and tailor stress distributions when using flywheels constructed from typical reinforced isotropic composite materials. For, in the case of isotropic materials, the stresses in a rotating flywheel of uniform thickness are dependent primarily on one material parameter, that is density. Plus, the stresses vary non-linearly with radius. Thus, in designing an isotropic flywheel of uniform thickness, the designer must establish an allowable speed based on not exceeding the allowable maximum stress, which maximum stress is a tangentail stress located at the inner edge of the wheel. Consequently, isotropic composite materials comprising a flywheel of uniform thickness are under utilized. Designers, in an attempt to make more effective use of the isotropic composite materials, vary the thickness of the flywheel, and proportion the flywheel so that the stresses are equal at every point in the flywheel. However, in filament wound isotropic composite assemblies, once the fibers and matrix materials are specified, the degree of tailorability of stress distributions is frozen.
Thus, it is an object of this invention to provide a composite flywheel wherein the various reinforcement elements (radial, circumferential, and axial) may be selected to design to provide, within a broad spectrum of possible variations, optimum levels of strength, stiffness, and density of provide maximum energy storage capability for the woven outer portion.
It is another object of this invention to provide a polar weave flywheel assembly that is capable of efficiently storing an amount of energy for a given weight and volume without failure, heretofore not possible in prior art flywheel assemblies.
It is also an object of this invention to provide a polar weave flywheel assembly having a hub-outer portion interface of enough strength to withstand failure due to external and internal forces associated with operation of the assembly.
It is still an object of this invention to provide a polar weave flywheel assembly with improved margins of safety for catastrophic failure and delamination.